Due to tolerances in the manufacturing process of photodiodes used in solid state imaging devices, there is usually a mismatch in sensitivity between pixels. In the case of a camera that takes pictures to be viewed by humans, the tolerances can be relatively low while creating an image of acceptable accuracy. A mismatch of around five percent is acceptable. However, for other applications such as cameras used for machine vision or optical mice, the allowable mismatch is much less, typically around one percent.
One way of improving such a mismatch is to remove it at the system level using calibration and compensation techniques. However, calibration requires special setup and also storage of the compensation coefficients. Compensation requires real-time processing, while costing silicon and consuming power. Hence, both these techniques are complex to perform and are expensive.
The areas of technology requiring an improvement in such a mismatch are the areas that are commercially expanding. Therefore, there is a need for a photodiode structure that provides improved matching between pixels.